Method for producing a sheet of paper comprising calcite

ABSTRACT

The invention relates to a method for producing a sheet of paper ( 6 ) comprising paper fibres and calcium carbonate mainly in the form of calcite crystals linked directly to the paper fibres. The inventive method is characterised in that it comprises the following steps:—(i) an aqueous composition comprising calcium bicarbonates ( 5 ) and/or hydrated and/or dissolved carbon dioxide ( 3 ) and (ii) an aqueous composition comprising calcium hydroxide ( 4 ) are mixed in an aqueous medium in such a way as to precipitate the calcium carbonate in the form of vaterite crystals;—paper fibres ( 2, 2 ′) are immediately added;—the vaterite crystals are left to transform into calcite crystals on contact with the fibres;—said mixture containing the calcite crystals which are fixed to the fibres is subsequently sent on the conveying wire of the papermaking machine (M) for drainage and so that the sheet of paper can be formed;—the sheet of paper ( 6 ) thus obtained is treated, if necessary, and dried.

This application is the national phase under 35 U.S.C. § 371 of PCTInternational Application No. PCT/FR02/00574 which has an Internationalfiling date of Feb. 14, 2002, which designated the United States ofAmerica.

The present invention concerns a process for the manufacture of a sheetof paper containing calcium carbonate in the form of calcite.

Various types of paper are known to contain mineral fillers, in order,on one hand, to reduce their costs, since the fillers are less expensivethan cellulose fibers, and, on the other hand, to provide or improvecertain physical and mechanical characteristics. Materials specificallyused as fillers are kaolin, talc, titanium oxide, aluminum hydroxide,satin white, and calcium carbonate in ground or precipitated form. Theclassic method for these fillers involves preparation ex situ at thetime of manufacture of the paper; they are incorporated into the paperfibers and retained through the addition of retention agents.

The development of paper manufacturing in an alkaline environment, inorder to make the paper more durable, has led to wider use of calciumcarbonate.

Calcium carbonate presents a number of crystallographic forms. The moststable and most commonly-used form is calcite; another, somewhat lessfrequently used form is aragonite; the least stable form is vaterite.Calcite and aragonite crystals are rhombohedral in shape, whereasvaterite crystals are spherical.

Paper-making processes have been proposed which enable the calciumcarbonate to be precipitated in situ onto paper fibers and fixed withoutthe addition of retention agents.

Such processes have been described in the patents listed below.

Patent application WO 9942657 describes a new process for the synthesisof calcium carbonate in the presence of cellulose fibers, so as toobtain, at the end of the process, a calcium carbonate precipitate insitu on the fibers. This process is characterized by the inclusion of:

-   -   a stage involving the realization of a first aqueous composition        containing calcium bicarbonate (also known as calcium hydrogen        carbonate), which can be obtained by processing calcium        carbonate with carbon dioxide;    -   a stage involving the realization of a second aqueous        composition containing calcium hydroxide; and    -   a final stage in which the first and second compositions are        mixed with cellulose fibers, in such a way as to cause the        precipitation of calcium carbonate upon contact with at least        some fibers.

When all of these three ingredients are present, the calcium carbonatecrystals are fixed onto the fibers, and a higher level of retention isobtained than when calcium carbonate is added to a paper compositionthat requires retention agents. This process is implemented in thedilute medium typically used in the paper-making process.

U.S. Pat. No. 5,679,220 also describes a method of fixing precipitatedcalcium carbonate onto paper fibers in a paper-making process usingcellulose fibers, calcium hydroxide and carbon dioxide under highshearing as a precipitating gas. The patent envisages a variation in themolar ratio of the carbon dioxide to the calcium hydroxide in order toobtain calcium carbonate crystals of different morphologies.Nonetheless, this patent does not clearly show how to obtain the desiredcrystallographic form of calcium carbonate.

The objective of the present invention is to provide a sheet of paperthat contains calcium carbonate in its common form of calcite and thathas improved “look-through.”

The applicant has shown that it is possible to act on crystallographicforms of calcium carbonate through the use of the in situ precipitationprocess of calcium carbonate, by adjusting the order and the durationsof contact between the various compositions involved, in order toprecipitate and fix the calcium carbonate onto the fibers in situ when asheet of paper is being manufactured. He has shown that the calciumcarbonate precipitated according to in situ processes initiallycrystallizes in the form of vaterite, which is unstable before beingtransformed into its more stable form of calcite, and that this passagefrom vaterite to calcite could be controlled and used at the time ofmanufacture of the sheet of paper, in order to improve the fixation anddistribution of the calcium carbonate crystals, in calcite form, withinthe paper.

The invention provides a process for the manufacture of a sheet of papercontaining paper fibers and calcium carbonate, mostly in the form ofcalcite crystals directly linked to the paper fibers, wherein saidprocess includes the following stages:

-   -   mixing, in an aqueous medium, of:        -   an aqueous composition containing calcium hydrogen            carbonates and/or hydrated and/or dissolved carbon dioxide,        -   an aqueous composition containing calcium hydroxide, so as            to precipitate calcium carbonate in the form of vaterite            crystals,    -   immediately adding paper fibers,    -   allowing the vaterite crystals to be transformed into calcite        crystals upon contact with the fibers,    -   placing this mixture containing the calcite crystals fixed to        the fibers onto the wire of the paper-making machine for        drainage and formation of the sheet of paper,    -   processing (if necessary) and drying of the sheet of paper thus        obtained.

Preferably, the paper-manufacturing process is characterized by the factthat the composition containing calcium hydroxide is added after thecomposition containing calcium hydrogen carbonates and/or hydratedand/or dissolved carbon dioxide.

Preferably, the paper-manufacturing process is characterized by the factthat the composition containing calcium hydrogen carbonates results froma mixture, in an aqueous medium, of recycled calcium carbonate andcarbon dioxide.

Preferably, the recycled calcium carbonate comes from recycled paperproducts, specifically recycled paper fibers and/or recycled whitewater.

Recycled white water originates in the water drained from the fibroussuspension on the wire of the paper-making machine. It contains fineelements, specifically fillers, such as calcium carbonate, and possiblyother alkaline or alkaline-earth compounds and cellulose fibers (called“fine” fibers) which are not retained on the wire, these fibersthemselves containing fillers. Like other possible sources of recycledpaper products, there are recycled paper fibers which originate in oldpapers, specifically de-inked and possibly bleached papers, and recycled“casse paper”; these products also contain fillers, such as calciumcarbonate and possibly other alkaline or alkaline-earth compounds.Another source might be de-inking sludge and other types of sludge fromthe paper-making process.

According to a particular embodiment of the invention, the carbondioxide is added in different places in the cycle of the paper-makingmachine. Preferably, this addition is performed before the addition ofthe calcium hydroxide, in order to ensure the dissolution of the gas andthus to subsequently encourage the speed of the reaction with thecalcium hydroxide in order to form the precipitated vaterite crystals.

Preferably, most of the carbon dioxide is added in the course of thewhite water recycling cycle of the paper-making machine. At the start ofthe cycle, the carbonic (carbon dioxide) gas participates in thesolubilization of the recycled calcium carbonate, in the form of calciumhydrogen carbonates, present in the white water. The calcium carbonatewill subsequently be recrystallized during reaction with the calciumhydroxide.

According to a particular embodiment of the invention, the process ischaracterized by the fact that the addition of said gaseous carbondioxide takes place between the white water receiving site under thewire of the paper-making machine and the mineralizer. This willencourage full dissolution of the gas before the reaction with calciumhydroxide. In fact, it is preferable for the carbon dioxide to be addedduring the recycling cycle of the white water, so that it is entirelydissolved, in free form, in hydrated form, or in the form of calciumhydrogen carbonates or other alkaline or alkaline-earth salts, dependingupon which of these ions are present.

An advantage of this process is that the fibers are not placed incontact with an alkaline product; this improves the homogeneity of thepaper, which is known as “look-through,” because the fibers have atendency to clump together in an alkaline environment. Duringrecrystallization of the carbonate in the presence of fibers, the mediumremains neutral; this improves the efficacy of the sizing agents and theoptical agents, which are alkaline pH-sensitive.

An advantage of this process, introducing the carbonic gas at thebeginning of the cycle (optimal dilution point for hydration of the gas)and in a machine adapted for mineralization of water, is that it iseasier to use a gas with a low CO₂ content, recovered from a boiler or alime kiln, in order to transform it into calcium hydrogen carbonate.

According to a particular embodiment of the invention, the process ischaracterized by the fact that the carbon dioxide is introduced in theform of diluted carbonic gas, specifically fumes from a boiler or alimekiln, containing 8% to 25% of CO₂.

In the process according to the invention, the molar ratio of carbondioxide to calcium hydroxide is equal to approximately 1, and istherefore stoichiometric.

Furthermore, the process according to the invention is alsoadvantageously characterized by the fact that the dilution rate of thepaper fibers in the final reaction mixture ranges from 0.1% to 5% byweight, preferably from 0.2% to 1.5%.

Preferably, the process according to the invention is characterized bythe fact that the composition containing the calcium hydroxide is anaqueous suspension of solid particles of said calcium hydroxide, knownas milk of lime.

More particularly, the process according to the invention ischaracterized by the fact that the calcium hydroxide is in the form ofan aqueous suspension of solid particles with a grading lower than 10μm, preferably between 0.5 and 2 μm, and particularly on the order of 1μm.

FIGS. 1 to 3 provide a diagram description of particular embodiments ofthis process, wherein the relative proportions are not shown in realscale.

FIG. 1 describes a process according to which a composition of calciumhydrogen carbonates (5) is introduced into an aqueous medium, to which acalcium hydroxide suspension (4) is added in order to precipitate thevaterite crystals, and, immediately afterward, virgin paper fibers (2)and/or recycled paper fibers (2′) are added and the vaterite crystalsare allowed to transform into calcite crystals upon contact with thefibers, after which the sheet (6) is shaped and drained on thepaper-making machine (M).

FIG. 2 represents a process according to which carbonic gas (3) isintroduced into the white water recycling cycle (1), after which acalcium hydroxide suspension (4) is added in order to precipitate thevaterite crystals, and, immediately afterward, virgin paper fibers (2)and/or recycled paper fibers (2′) are added and the vaterite crystalsare allowed to transform into calcite crystals upon contact with thefibers, after which the sheet (6) is shaped and drained on thepaper-making machine (M).

FIG. 3 represents the simplified diagram of a detailed, non-limitativeexample that describes the manufacture of a paper (6) with a 26% fillerof calcium carbonate, principally in the form of calcite, in thefollowing manner:

Let us assume that an industrial installation manufactures 5.4 metrictons of paper per hour, characterized by the following operatingconditions for the principal cycles:

-   -   Average flow rate of the suspension into the headbox: 1100 m³        per hour    -   Average flow rate of the short recycled water cycle: 1000 m³ per        hour    -   Average flow rate of the input cycle of virgin fibers and        recycled paper: 100 m³ per hour    -   Water temperature in the cycle: 40° C.        The average composition of the suspension of virgin fibers (2)        and recycled paper (2′) consists of 28 g/L of virgin fibers and        16 g/L of recycled paper coming from the same production, which        itself includes 12 g/L of fiber and 4 g/L of calcium carbonate        (recycled). The average composition of the recycled white water        (1) consists of 1 g/L of fiber and 1 g/L of calcium carbonate        (recycled).        Upstream of the pump (P) for recovering the white water (1)        recovered under the wire of the paper-making machine, a quantity        of carbonic gas (3) equal to 440 kg/hour of CO₂ is initially        introduced into the closed short cycle. Then, after the        mineralizer (R), milk of lime (4), containing 100 g/L of very        fine-grained (approximately 1 μm) calcium hydroxide is added at        a flow rate of 7.4 m³ per hour. Finally, the suspension        containing virgin fibers (2) and recycled paper (2′) is        introduced.

The gas reacts almost instantaneously with the milk of lime, in order toform, in suspension in the water, unstable vaterite crystals betweenpoints a and b; subsequently, after being mixed with the fibroussuspension (2, 2′) and before reaching the headbox (E), the vateritecrystals are transformed into stable calcite. The suspension of fibersand fillers is then conveyed onto the wire of the paper-making machine(M) for drainage and formation of the sheet of paper.

When the virgin fibers and the recycled paper are added, the pH of thesuspension is stabilized at its final value, adjustable between 7 and 8as desired, which corresponds to a calcium-carbon balance of the water.

The output of the paper-making machine consists of 5.4 tons of paperwith a 26% filler of crystals, essentially in the form of rhombohedralcalcite.

The size of the calcite crystals becomes smaller as the grading of themilk of lime becomes finer. The paper obtained will accordingly be moreopaque when the milk of lime used contains grains less than 1 μm indimension.

The high opacity of the sheet (6) is obtained by virtue of the veryconsistent distribution of the crystals hooked on to the fibers, withoutthe help of a retention agent and without any variation in the pH. Incomparison with the traditional process of filler retention withformation of aggregates by means of a retention agent, the improvementin opacity is between 3 and 4 points.

EXAMPLE FROM THE PRIOR ART

A sheet of paper is made according to the prior art whereby calciumcarbonate is precipitated in situ, mixing the compounds at the same timewithout specifically adding the paper fibers last, and without addingthem immediately after implementing the precipitation of the calciumcarbonate.

It will be noted that the sheet of paper obtained according to theinvention contains calcite crystals that are better distributed thanthose in the sheet of paper prepared according to the prior art, andthat, accordingly, the “look-through” of said sheet of paper obtainedaccording to the invention is improved.

1. A process for the manufacture of a sheet of paper containing paperfibers and calcium carbonate, mostly in the form of calcite crystalsdirectly linked to the paper fibers, wherein said process includes thesteps of: providing an aqueous composition containing calcium hydrogencarbonates and/or hydrated and/or dissolved carbon dioxide; addingthereto an aqueous composition which is an aqueous suspension of solidparticles of calcium hydroxide, known as milk of lime, so as toprecipitate calcium carbonate in the form of vaterite crystals, whereinthe calcium hydroxide is in the form of solid particles with an averagegrading between 0.5 and 2 μm; immediately adding paper fibers; allowingthe vaterite crystals to be transformed into calcite crystals uponcontact with the fibers; placing this mixture containing the calcitecrystals fixed to the fibers onto the wire of the paper-making machinefor drainage and formation of the sheet of paper; and processing (ifnecessary) and drying the sheet of paper thus obtained.
 2. The processof claim 1, wherein the composition containing calcium hydrogencarbonates results from a mixture of recycled calcium carbonate andcarbon dioxide.
 3. The process of claim 2, wherein the recycled calciumcarbonate comes from recycled paper products.
 4. The process of claim 1,wherein the carbon dioxide is added before the addition of the calciumhydroxide, in the white water recycling cycle of the paper-makingmachine.
 5. The process of claim 1, wherein the carbon dioxide isintroduced into said aqueous composition in the form of diluted carbonicgas.
 6. The process of claim 1, wherein the dilution rate of the paperfibers in the final reaction mixture ranges from 0.1% to 5% by weight.7. The process of claim 1, wherein the sheet of paper obtained includesat least 10% of calcium carbonate crystals in the form of rhombohedralcalcite.
 8. The process of claim 3, wherein the recycled calciumcarbonate comes from recycled paper fibers and/or recycled white water.9. A process for the manufacture of a sheet of paper containing paperfibers and calcium carbonate, mostly in the form of calcite crystalsdirectly linked to the paper fibers, wherein said process includes thesteps of: providing an aqueous composition containing calcium hydrogencarbonates and/or hydrated and/or dissolved carbon dioxide, wherein thecarbon dioxide is introduced into said aqueous composition in the formof diluted carbonic gas, which diluted carbonic gas is in the form offumes from a boiler or a lime kiln containing 8% to 25% of CO₂; addingthereto an aqueous composition which is an aqueous suspension of solidparticles of calcium hydroxide, known as milk of lime, so as toprecipitate calcium carbonate in the form of vaterite crystals, whereinthe calcium hydroxide is in the form of solid particles with an averagegrading between 0.5 and 2 μm; immediately adding paper fibers; allowingthe vaterite crystals to be transformed into calcite crystals uponcontact with the fibers; placing this mixture containing the calcitecrystals fixed to the fibers onto the wire of the paper-making machinefor drainage and formation of the sheet of paper; and processing (ifnecessary) and drying the sheet of paper thus obtained.
 10. The processof claim 6, wherein the dilution rate of the paper fibers in the finalreaction mixture ranges from 0.2% to 1.5% by weight.
 11. The process ofclaim 1, wherein the calcium hydroxide is in the form of solid particleswith an average grading on the order of 1 μm.
 12. The process of claim7, wherein the sheet of paper obtained includes at least 20% of calciumcarbonate of the calcite type thus precipitated.
 13. The process ofclaim 4, wherein the carbon dioxide is introduced into said aqueouscomposition in the form of diluted carbonic gas.
 14. A process for themanufacture of a sheet of paper containing paper fibers and calcitecrystals directly linked to the paper fibers, said process comprisingthe steps of: introducing diluted carbonic gas in the form of fumes froma boiler or a lime kiln containing 8% to 25% of CO₂ into an aqueouscomposition to provide an aqueous composition containing calciumhydrogen carbonates and/or hydrated and/or dissolved carbon dioxide;adding, to the resulting aqueous composition containing calcium hydrogencarbonates and/or hydrated and/or dissolved carbon dioxide, an aqueouscomposition which is an aqueous suspension of solid particles of calciumhydroxide in the form of solid particles with an average grading between0.5 and 2 μm to precipitate calcium carbonate in the form of vateritecrystals; immediately adding papermaking fibers to the resulting aqueouscomposition containing vaterite crystals; allowing the vaterite crystalsto be transformed into calcite crystals fixed to said papermakingfibers; and placing the resulting aqueous mixture containing calcitecrystals fixed to papermaking fibers onto a wire of a papermakingmachine to form a sheet of paper, wherein the sheet of paper thusobtained includes at least 20% of calcium carbonate of the calcite typedirectly linked to the paper fibers.